This invention relates to papermaking and more particularly to a system for controlling the thickness and sheet quality of paper passing through calendar rolls.
One of the final stages in the papermaking operation is a calendering operation where a dry web of paper is passed through a narrow nip formed between two calender rolls. The calendering operation takes place downstream from the location where the web is formed, and it is performed by a calender stack consisting of two or more contacting, rotating metal rolls. The web is threaded through the rolls and exposed to the contact pressure generated by the contact of adjacent rolls. The contact pressure alters the web thickness and surface qualities, and the alteration of these web qualities can be regulated by a controlled variation of the roll-to-roll contact pressure. The interaction of the rolls and the web result in compacting the web and changing the caliper, density and surface and optical characteristics of the web by pressure, friction, temperature and other physical condition changes. The uniformity of the compacting action, or calendering intensity, depends on the uniformity of the nip pressure, which depends on the uniformity of the contact between the adjacent rolls, and which in turn depends on the local roll diameter (the diameter of a portion of the roll).
Provided that the rolls are made of materials that respond to changes in temperature, the roll-to-roll contact pressure can be altered by causing changes in temperature along the roll to thereby effect a controlled variation of the local roll diameter. The local roll diameter can be altered by either controlled local heating or local cooling of the roll which results in an expansion or contraction of the local roll diameter respectively.
There are several known methods for altering the local roll diameter by local heating or cooling. Friction pads have been pressed against a local area of the roll to raise the temperature and thereby increase the local roll diameter. Such pads, however, tend to wear the roll surface and thus defeat the purpose for which they were intended.
Single nozzle air-jets have also been employed for many years (typically utilizing compressed or low pressure air) to change the temperature and thereby the local roll diameter in the region of air application. The magnitude of correction and response achieved through the use of such single-jet "air showers" is low because of the low effective heat-transfer rate over the desired surface area to be controlled. This low effective heat-transfer rate results from the fact that the high jet-velocity originating from the nozzle strikes the roll surface over a small localized region of the roll. The velocity-vector following impact is then roughly parallel to the roll surface, and exhibits a relatively low heat transfer rate when compared to the original impingement-flow heat transfer rate. The effective heat-transfer rate over an arbitrary portion of the roll surface larger than the original area of jet impact is then significantly lower than desired. In addition, due to the curvature of the roll and the rebounding of jet-air after impact, the entire area of the roll which is to have its diameter changed does not come into contact with the heat-transfer fluid. As a result, the control of the local roll diameter through the directing of a single cross-machine row of jets of hot or cold air against the local area of the roll is, therefore, not entirely adequate (especially when the high cost of energy and the considerable amounts of energy required to cause an acceptable change in the local roll diameter are considered).
Some systems have employed the use of a "shroud" through which the nozzle air projects to keep the "spent-flow" in contact with the roll over a significantly larger portion of the area to be controlled. While systems utilizing such shrouds are more efficient than single-nozzle air-jet systems, the effective heat-transfer rate of systems utilizing a shroud is still hampered by the relatively low heat transfer rates coincident with parallel-flow as is observed over the control area contacted by the spent-flow.
A tremendous amount of energy can be wasted in heating or cooling the rolls because with typical system efficiencies, the changing of the roll surface temperature a chosen number of degrees requires application of a fluid of a temperature considerably lower or greater than the desired roll surface temperature, and the energy utilized to effect the temperature change is lost once the fluid is applied to the roll. It is therefore important that the percentage of the energy consumed in the creation of the temperature difference, which is transferred to or from the roll surface, is minimized.
Another limitation of known caliper control equipment is that the devices used for heating or cooling the roll should be of a size capable of being placed adjacent a roll while at the same time leaving enough space for other equipment to be positioned in a working relationship with respect to adjacent rolls. With reference to the equipment commonly used to initially heat or cool the air, the heat-transfer rates (as related to the convective heat-transfer coeffecients of the equipment) offered by conventional steam-to-air or water-to-air heat exchangers, are prohibitively low, so as to render the size of such exchangers unacceptably large for their installation in the immediate region of the calender stack. When reducing equipment size, however, it is important to insure that the absolute magnitude of heat-transfer to or from the roll is satisfactory and therefore not greatly reduced.
Most existing caliper-control systems employ one of the four following control methods:
(1) Only heating of the roll is performed, as with hot air or induction heating. PA1 (2) Only cooling of the roll is performed with cold air. PA1 (3) Heating and cooling of the roll is performed with one unit, the nozzles of which pass hot or cold air as desired from one of two supply chambers which are housed together within the same apparatus. PA1 (4) Heating and cooling of the roll is performed by applying a uniform air-flow against the roll, across the full roll width, the temperature of which is positionally controlled from the ambient supply temperature up to a suitable maximum.
Each of the above-described methods possess certain disadvantages. When a caliper control device can only heat a roll, only expansion of the roll and the related further sheet compression can be executed directly. To profile a sheet that exhibits both thin and thick profile inconsistensies across the machine, it is necessary to establish a roughly 50% output baseline for the system. In other words, for those sheet positions where the thickness requires no correction, the actuators or nozzles would operate at approximately 50% power output or heat-transfer rate. For those sheet positions that are too thick, requiring additional compression, the nozzles would operate anywhere from approximately 50% to 100% output, as required. For those positions that are too thin, requiring that the compression be disminished, the nozzles would operate anywhere from approximately 50% down to 0%, as required.
In the heating mode (50 to 100% output) the sheet response is relatively rapid due to the "forced" heat-transfer to the roll provided by the actuators or nozzles. In the cooling mode, however, (0 to 50% output) the time response of the system is limited to the speed at which the related roll position can expel its thermal energy through such phenomena as natural convection, radiation, etc. The cooling-mode response is obviously significantly slower than the heating-mode response.
As "heating only" caliper control systems are typically part of a closed-loop control systems, including an on-line sheet-thickness scanner and a computer station which analyzes the sensed values and requests action by the caliper-control system accordingly, the response of the differential control loop is only as rapid as the slower of the two heat-transfer modes, which severely hampers the settling-time of the control effort. In addition, because the system base-line or "zero-point" is approximately 50% output, in the presence of a majority of sheet positions which require little or no control, the average operating power consumption is unnecessarily high.
In "cooling only" caliper control systems, only contraction of the roll and related reduction of the sheet compression can be executed directly. The disadvantages of this approach are identical to those of the heating only system described above, but opposite in nature.
In heating and cooling caliper control systems, expansion and contraction of the roll can be effected directly. These systems may utilize either of two types of in-line nozzles. One known nozzle includes two individual nozzles - one for imparting cold air from a cold air supply plenum and another for imparting hot air from the hot air supply plenum. The other known type of nozzle comprises a single nozzle which selectively imparts air from either of the two plenums, as required. With both types of nozzles, conduction from one plenum to the other through the body of the common housing decreases the effective temperature gradient available from both supplies. In addition, with the second type of nozzle, a nozzle previously imparting hot air exhibits a substantial thermal time-lag when requested to revert to cooling operation. The same problem is faced in the opposite situation of a nozzle operating in the cooling-mode being requested to change to the heating-mode.
Systems which apply a constant air flow of variable temperature typically utilize compact electrical resistance heaters, located individually in each nozzle-outlet region, to positionally control the nozzle exit temperature. Such resistance heaters exhibit a thermal time-lag when requested to revert from high to low temperature operation, or visa-versa, the effect of which is to hamper the response and settling time of the profiling effort.
A steel roll (as is commonly in use on a calender stack) exhibits, when locally cooled or heated, a tendency to expand or contract less than would be the case for a uniformly heated or cooled body because of the existence of built-up thermal stresses which oppose the radius change. In addition to the undesirable, but unavoidable, radial temperature gradients which limit the radial change in response to a surface temperature change, axial surface temperature gradients, which result when one region of the roll surface is heated or cooled more than an adjacent region, also reduce the effective radial change at the desired location. Often, when a thin and thick sheet condition are close in axial proximity, heating and cooling must be executed in nearly adjacent surface locations, which reduces the effective radius change capability of each action because of the resultant axial surface temperature gradients.
Present caliper-control systems typically utilize large quantities of electrical energy for the purpose of imparting heat to the roll. Typical rates of consumption of 5 to 10 kw per cross-machine foot are common. A system employing three inch spaced nozzles on a three-hundred inch machine would thus require 100 separate power circuits accounting for 1.25 to 2.5 kw each. In addition to the fact that such electrical circuitry may be considered complex, electrical energy in some regions of the world is prohibitively expensive.
Systems which employ heated or cooled air often preheat or precool the air at a distance from the caliper-control unit (usually in close proximity to the air supply fan). The conduits which convey the air to the caliper-control unit must therefore be insulated to prevent undesirable heat-losses or gains, to or from the air, between the heating and cooling exchangers and the caliper-control unit. The initial costs, as well as the installation costs, of such insulation may be substantial.
Finally, the accurate and repeatable control of heat-transfer rates achieved through the use of air nozzle type systems is difficult to accomplish for a number of reasons. Often, the heat-transfer rate is varied by altering of the volumetric flow-rate. Problems, however, result because the flow-rate is typically not linearly-proportional to the travel of the actuator component used to modulate the flow and because the heat-transfer rate is typically not linearly-proportional to the air flow rate.
It is therefore a principal object of the invention to provide a system and method for heating and cooling a desired local area of one or more calender rolls in an effecient manner, using a minimal amount of readily available low-cost energy, in a compact form, with accurate, repeatable, and linearly-adjustable control.
Another object of the present invention is to provide a system and method for heating or cooling only one local area of a roll by applying a uniform flow of air to only that one local area of the roll.
A further object of the present invention is to provide a system and method for heating or cooling a selected local area of a roll which maintains the high heat-transfer rates available from impingement flow over the full area of the surface intended to be controlled.
Another object of the present invention is to provide a system and method for heating or cooling a selected local area of a roll which exhibits an impingement flow pattern which optimizes the heat-transfer to energy-consumed ratio exhibited by the apparatus.
Yet another object of the present invention is to provide a system and method for varying local roll diameter in which the effectiveness of the heating and cooling modes, when executed in close cross-machine proximity to one another, can be improved by applying the two heat-transfer modes to different calender rolls, thereby reducing the resultant axial temperature gradients with respect to any one roll.
Still another object of the present invention is to provide a system and method for varying the local diameter of a calender roll which provides for the "forced" heating and cooling of separate rolls, the roll temperatures being opposite in sense to the temperatures of the supply air utilized to vary the diameter.
A still further object of the present invention is to provide a system and method for varying the local diameter of a roll which can achieve high enough heat-transfer rates to allow for the use of lower temperature heating-mode supply air, which in turn enables the use of steam as the energy source, without detrimentally limiting the absolute nozzle-to-roll heat-transfer rate.
It is another object of the present invention to provide a system and method for varying the local diameter of a roll which heats or cools the air applied to the roll at the calender stack itself so as to eliminate the need for conduit insulation, and thereby minimize the likelihood of undesirable heat-losses or gains to or from the hot and cold air prior to its application to the process.
An even further object of the present invention is to provide a heat-exchanger whose design and size enables the exchanger to be installed in the immediate region of the calender stack.